:The output signals don't go straight from the chip - do they?
:I've seen few KOhms resistors on most boards for each output pin.
:
:27.10.2002; 19:32:00
:[SorAlx] http://cydem.zp.ua/
It depends on the chip. Most modern serial driver chips have series
resistors on the outputs, inside the chip, and do not need any external
current limiting (which is what the resistor does in effect). Parallel
port chips typically do not have output resistors and instead depend
on beefy TTL output stages, and a good motherboard manufacturer will put
series resistors on those pins. All modern chips have diode protection
on their outputs but you can still zap them (static shock it to death).
Serial pins tend to be far, far more robust then parallel port pins
due to having to go through level converters to get the right voltages
on outputs and to be able to handle +/-12V on inputs. For example,
most serial driver chips can handle up to +/-25V on their inputs before
they blow up. Try putting even +12V on a parallel port pin and you
will likely fry the chip, even with an external series resistor, and
an external series resistor alone will not stop a big static shock,
only a Zener diode is fast enough to do that. An external series
resistor is there strictly for current limiting purposes.
-Matt
Matthew Dillon
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